In present day data-processing systems, magnetic tape decks are commonly used because they have large storage capacity, and require relatively short times for magnetic read/write heads to access data contained anywhere on the magnetic recording tapes from the moment when the heads receive a data access command from the processing system. Magnetic tapes carry data, in coded (usually binary) form, on parallel recording tracks (usually nine) having widths that do not exceed a few hundredths of a millimeter. It is current practice for tape decks to carry only a single removable magnetic tape which is replaced by another tape as soon as reading and/or writing operations involving the first tape have been completed.
Each read-out track of a tape has associated with it a read/write head which is positioned very close to, or even in contact with, the track. The tape moves discontinuously past an assembly formed by the heads for all of the recording tracks. The discontinuous movement is a sequence of "unitary movements", each comprising: (a) a tape speed up phase, during which the tape has a high acceleration; (b) a phase during which the tape moves at a substantially constant speed V.sub.O, and (c) a braking phase during which the tape has a high deceleration until it is completely stopped. It is current practice for the data to be read, during each "unitary movement", after the speed up phase, while the tape is moving at constant speed V.sub.O.
What are termed slow variations in the tape speed are speed variations about speed V.sub.O which take place while the data are read. Frequently these variations are .+-.25% of the speed V.sub.O and somtimes as great as .+-.50% of V.sub.O. The duration of these variations is a few fractions of a millisecond. Instantaneous variations in the tape speed, on the other hand, are speed variations having durations approximately a hundred to a thousand times shorter than that of the slow variations.
When binary data bits recorded on a magnetic tape pass by an assembly of magnetic read/write heads associated with all of the recording tracks, each of the heads derives a series of analogue electric signals which are shaped into a series of square-wave electrical pulses by shaping circuits. The pulse voltage varies between minimum and maximum values v.sub.min and v.sub.max. For ease of exposition, a description will be given only to the signals derived by a single head; it is to be understood that the same description is equally applicable to the signals derived by the oter heads. The leading edge of the electrical pulse is that part of the pulse during which the voltage changes from the value V.sub.min to the value V.sub.max. Oppositely, the trailing edge of a pulse is that part of the pulse during which the voltage changes from the value v.sub.max to the value V.sub.min. The binary codes most frequently used in writing data on magnetic tapes are such that, after the signals have been read and shaped, a bit equal to "logic one" corresponds to a leading or positive going edge of a pulse while a bit equal to "logic zero" corresponds to the trailing or negative going edge of a pulse.
The series of transduced square-wave electrical pulses constitutes a substantially cyclic signal DE having a nominal mean frequency F.sub.O and period T.sub.O ; hence, T.sub.O defines a single signal bit or "bit cell period". It is clear that the frequency F.sub.O of the transduced signal is porportional to the tape speed. Hence, thehigher the tape speed, the greater the number of data items read by the magnetic head per unit of time, whereby frequency F.sub.O corresponds to speed V.sub.O. For any variation in the tape speed there is a corresponding frequency variation. Thus for a slow tape speed variation there is a corresponding low frequency and for an instantaneous speed variation there is a corresponding instantaneous frequency variation.
If t.sub.0 is the time at which a given "bit cell" begins, the time (t.sub.0 +T.sub.O /2) is termed the "center of the bit cell" and the time (t.sub.0 +T.sub.O) is termed the "end of the bit cell". Each cell contains either a leading or trailing pulse edge situated in the center of the cell, and possibly, a leading or trailing edge at the end of the cell. Only rising or decaying edges situated in the center of "bit cells" are considered to represent bit values.
A transduced signal DE is supplied to an apparatus for detecting the data recorded on the magnetic tape of the tape deck. Such a detecting apparatus determines the value of each of the data bits recorded on magnetic tape and operates in three phases. During phase one, all the leading or trailing pulse edges in the center of each cell bit of signal DE are recognized to determine the value of the data bits. During phase two, each of the recognized edges is converted into a signal having an amplitude that remains constant during the period T.sub.O of this cell. A leading edge is converted into a signal of constant positive amplitude which is termed a "high level", whereas a trailing edge is converted into a signal of constant negative amplitude which is termed a "low level". The positive and negative amplitude signals are referred to collectively by the name "signal DEI". During phase three, the value of the bit corresponding to each cell is determined from signal DEI during each period T.sub.O. High and low levels, respectively, correspond to bit values of one or zero.
Imperfections in the magnetic tape and magnetic reading heads, as well as slow and instantaneous tape speed variations, cause distortion in both the amplitude and the phase of the signals read by the head, so that the signal amplitude is reduced and is phase shifted. The distortion is increased by the electronic shaping circuits and the data detecting apparatus and is manifested as a shift in the time position of the edges at the beginning or center of the bit cell. It can further be shown that the distortion increases as the density of the data recorded on the magnetic tape increases, that is, as the number of data items recorded per unit of length of the magnetic tape increases. The phase and amplitude distortion of signals DE and DEI may be relatively severe.
In the prior art there are simple and effective magnetic tape data-detecting devices which enable data bits to be detected with very great accuracy despite considerable phase and amplitude distortion in the signal DEI. Such an arrangement is described in French Pat. No. 2,138,029 entitled "Method and apparatus for detection by integration" filed May 17, 1972 by the S.T.C. company. In this prior art device the bit values are determined (phase 3) by a pair of integrators, each of which integrates the high and low levels of signal DEI during the period T.sub.O of the corresponding bit cell, which defines an integrating period.
To ascertain the bit value, it is merely necessary to determine, at the end of each period T.sub.O of one bit cell, the polarity of the integrated signal, designated DEINT. If signal DEINT is positive, or negative, the bit value is respectively equal to one or zero. After each integration operation, the integrating arrangement is returned to an initial, rest state during which the integrator output remains constant so that the sign of the integrated signal DEINT may be accurately determined from a constant reference level. Although the term is not strictly correct, the integrating arrangement is said to be reset to zero. If there were no zero reset, errors could occur when determining the polarity of DEINT and thus when determining the value of the data bits.
The integrating arrangement described in French Pat. 2,318,029 includes an integrator containing a capacitive integrating member C responsive to a charging current derived by a constant current generator regardless of the frequency F.sub.O of the signal DEI to be integrated. A device for controlling the integrator responds to signal DEI to control the direction and duration of the flow of the charging current supplied to the capacitive member C whereby the current flows during the period T.sub.O and the polarity of the integrated signal DEINT at the terminals of the capacitive member at the end of the integrating period T.sub.O is the same as signal DEI. A zero-reset circuit for the integrator resets the integrator to zero at the end of each integrating operation.
Since the voltage V.sub.C of signal DEINT, at the terminals of capacitive member C, is a linear function of time and the charging current is constant, V.sub.C.sbsb.T =kT.sub.O =k/F.sub.O ; where V.sub.C.sbsb.T =the value of V.sub.C at T.sub.O, k=I/C, and I is the constant charging current. Hence, voltage V.sub.C.sbsb.T varies as a function of the frequency F.sub.O of signals DE and DEI and is therefor a function of the slow or instantaneous variations in the magnetic tape speed. The effects of these variations on the voltage V.sub.C, added to those of the phase and amplitude distortion already mentioned, reduce the accuracy of the integration and thus the accuracy with which the values of the data bits are determined.